This invention relates in general to the field of radio frequency (RF) receivers and more particularly to direct conversion receivers that include direct current (DC) offset correction.
One type of radio receiver known in the communications arts is the direct-conversion receiver (also referred to as a “homodyne” or “zero-frequency” receiver.) FIG. 1 shows a front-end portion 10 of a typical direct-conversion receiver. An antenna 100 receives an RF signal, which is then typically filtered by a band-pass filter 102 to remove strong out-of-band signals. The filtered signal is then received by a balun 104, which splits the filtered signal and provides impedance matching between filter 102 and a low-noise amplifier (LNA) 106. LNA 106 then amplifies the split signal. A local oscillator 108 provides a reference signal having a frequency that is nominally centered at the channel frequency being received. A quadrature generator 110 operates on the local oscillator signal to generate an in-phase signal and a 90-degree out-of-phase signal. Balanced mixers 112 and 114 receive the in-phase and out-of-phase signals from quadrature generator 110, respectively. Mixers 112 and 114 also receive the amplified signal from LNA 106. Upon receipt of these signals, mixers 112 and 114 generate an in-phase output signal (I-channel) and an out-of-phase (or “quadrature”) output signal (Q channel). In effect, mixers 112 and 114 are multiplying devices, which generate both sum frequency components centered around two times the carrier frequency and difference frequency components centered around zero frequency. Low pass filters 116 and 118 operate to filter out the sum frequency components, thereby leaving the downconverted difference frequency components centered around zero frequency in both the I and Q channels.
A problem characteristic of the direct-conversion receiver relates to the downconverted band being centered around zero frequency. Centering of the downconverted band around zero frequency can be problematic, since any extraneous DC offsets may be unwantedly amplified in the I and Q channels. The amplified DC offsets can corrupt the received signal and can even impede reception of the receiver if the DC offsets are amplified to levels that saturate subsequent stages of the receiver. Because of these problems, DC offsets in a direct-conversion should be removed or reduced to acceptable levels.
U.S. Pat. No. 5,241,702 discloses a method of removing DC offsets by digitizing the time derivatives of the received waveform. By using the derivative of the received signal, the DC offsets are removed. Once they are removed, the differentiated signal is amplified and integrated. The integration essentially restores the filtered components to their original values. Using various techniques that exploit predetermined signal patterns or inherent signal properties of the desired signal, the DC offset estimate is then subtracted out of the restored signal, thereby leaving the amplified, received signal substantially free from distortion. A disadvantage of this method, however, is that removal of distortion in the signal path requires complimentary and equalized filter matching.
Another solution to removing the DC offsets is to provide AC coupling in the signal path. However, this approach is also undesirable for a number of reasons. First, to prevent unrecoverable loss of the signal in narrow-band applications, the cutoff frequency of the coupling must be very small. This requires a very large capacitor, which can lower the response time of the receiver. Moreover, introduction of a large capacitor in a receiver design implemented as a monolithic integrated circuit is unfavorable, since large capacitors consume large areas of the integrated circuit implementation. Yet another reason AC coupling is undesirable is that many applications such as, for example, wireless communications, often use a burst-type protocol where a data pattern may emulate a DC level for an extended period of time. AC coupling, in such circumstances, would operate to remove the data pattern, thereby resulting in an unwanted corruption of the data.